An audio frequency detection device is usually installed in a protected area in order to sense instrusion and subsequently trigger an alarm unit by detecting high-frequency sound such as that of breaking glass. The traditional audio frequency detection device has a major defect in adjustment of the device's sensitivity, however. If the sensitivity is set too low, a thief can easily enter the protected area undetected; if the sensitivity is set too high, ordinary environmental noise may be picked up by the device and generate a false alarm. Therefore, the user must usually opt to set the sensitivity of the device at an intermediate position, which is frequently not the ideal setting.
In order to eliminate the above defect, the present inventor has created an environmental interference detection device to allow the audio frequency detection device to be adjusted to an optimum sensitivity level.